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Bacteriophage – Lytic and Lysogenic Phage

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Abstract
Viruses that parasitize the bacteria are called bacteriophages.Most studied bacteriophages are T2, T4, and T6. Bacteriophages are generally of two symmetries which are cubical and helical. Cubical bacteriophages are regular or icosahedral having 20 faces.Helical bacteriophages are rod-shaped. The head of bacteriophage is hexagonal, prism-shaped structure. Within head there is present double stranded DNA. The tail is sheath comprised of protein.

The lytic cycle consists of:
1) attachment: attach to receptor site on bacterial cell wall by weak linkage
2) penetration: tail release enzyme lysozyme to dissolve bacterial cell wall. Phage injects its DNA into cell wall
3) replication: viral DNA takes control on hosts biosynthetic machinery
4) lysis: bacteria undergo lysis. The phage which causes lytic cycle is called lytic or virulent phage.

The phage which causes lysogeny is called temperate or lysogenic phage.

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What are bacteriophages?

A bacteriophage, also simply called a phage, is a type of virus infection that contaminates bacteria. In fact, the word “bacteriophage” literally implies “bacteria eater,” due to the fact that bacteriophages destroy their host cells.

Structure of Bacteriophage

All bacteriophages are composed of a nucleic acid molecule that is surrounded by a protein structure. Bacteriophages occur in 2 structural types having cubical or helical symmetry. In general look cubical phages are regular solid or icosahedral (having 20 faces), and helical phages are rod formed. Many phages consist of head and tail. In those cases, heads are polyhedral however tails are rod-shaped.

The T Phage

Earlier researches on bacteriophages were mainly on a restricted number of phages that infect Escherichia coli. Of these the very best-known phages are T phages (T for type).

Amongst T phages, the T2 and T4 phages are primarily used in phage studies.

Structure of T Phage

The overall structure of T4, studied with electron microscopy, resembles that of a tadpole, consisting of head and tail. The head is an extended pyramidal (having two triangular structures with a typical base), hexagonal, prism-shaped structure, to which a straight tail is connected. Within the head double-stranded DNA molecule is present.

The structure of the phage tail is more complicated than the head. A layer of distinct protein forms the inner tube or core, which is confined in a sheath comprised of another type of protein. On one side of the sheath is the collar and on the opposite is the end-plate. To the end-plate six tail fibers are connected, which are the structures for attachment. The volume of the phage is about 1/1000 of the bacteria (host).

Structure-of-T-Phage

Life Process of T Bacteriophage

The bacteriophage duplicates only inside the bacterial cell. The initial step in the replication of a bacteriophage is its attachment (adsorption) to the host cells at the receptor site on the cell wall of the bacterium. During attachment, the week chemical union between virion and receptor site takes place.

In the next step, penetration, the tail releases the enzyme lysozyme to dissolve a portion of the bacterial cell wall. The tail sheath contracts and the tail core is pushed into the cell through the cell wall and cell membrane. The bacteriophage injects its DNA into the cell just as the syringe is utilized to inject the vaccine. The protein coat, which forms the phage head and tail structure of the virus remain outside the cell. Many animal viruses, however, get in the host cell as a whole.

Lytic or Virulent Phage

Instantly after entering the host cell, the viral nucleic acid takes the control of the host’s biosynthetic equipment and induces the host cell to synthesize necessary viral components (DNA, proteins), and begins multiplying. About 25 minutes after preliminary infection, around 200 brand-new bacteriophages are formed, bacterial cell bursts, i.e., it goes through lysis. Freshly formed phages are released to infect the bacteria and another cycle, the lytic cycle starts. The phage which causes lysis of the host cell is called lytic or virulent phage.

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Virulent-Phage

Lysogenic or Temperate Phage

All infections of bacterial cells by phages do not lead to lysis. In some cases, viral DNA, instead of taking over the control of the host’s equipment, ends up being integrated into the bacterial chromosome. Phage in this state is called prophage and this procedure is called lysogeny. In this condition, the bacteria continue to live and replicate normally.

Viral DNA being the part of bacterial chromosome passes to each daughter cell in all succeeding generations. Sometimes, nevertheless, the viral DNA gets detached from the host’s chromosome and the lytic cycle starts. This process is called induction. Lysogenic bacteria are resistant to infection by the same or associated phages. The phage which triggers lysogeny is called temperate (lysogenic) phage.

Temperate-Phage 

Uses of Bacteriophages
  • Bacteriophages (BPs) are viruses that can infect and kill bacteria with no harm or hazardous impact on human or animal cells. For this reason, it is expected that they can be utilized, alone or in the mix with prescription antibiotics, to deal with bacterial infections.
  • Bacteriophages can be extensively applied in the food industry. They can be used for the protection of food at the pre- and postharvest phase, as preservatives, to extend the expiration date of food, and to keep clean the surface areas of equipment utilized in the production of plants.

 

FAQs (Frequently Asked Questions) – Bacteriophage – Lytic and Lysogenic Phage

  1. What are bacteriophages?
    • Bacteriophages, commonly known as phages, are viruses that infect and destroy bacteria. The term “bacteriophage” literally means “bacteria eater.”
  2. What is the structure of a bacteriophage?
    • Bacteriophages have a nucleic acid molecule surrounded by a protein structure. They come in two structural types: cubical or helical, with cubical phages having icosahedral symmetry and helical phages being rod-shaped.
  3. What is the T Phage?
    • T Phages, particularly T2 and T4, are well-known bacteriophages extensively studied in phage research. They infect Escherichia coli bacteria.
  4. How does the life process of a T Bacteriophage occur?
    • The life process involves attachment to the host cell, penetration through the bacterial cell wall, injection of DNA into the cell, replication using the host’s biosynthetic machinery, and lysis of the bacterial cell.
  5. What is a Lytic or Virulent Phage?
    • A lytic or virulent phage is one that causes the lysis of the host bacterial cell. It takes control of the host’s biosynthetic machinery, induces the synthesis of viral components, and results in the release of new bacteriophages.
  6. What is a Lysogenic or Temperate Phage?
    • A lysogenic or temperate phage does not immediately cause lysis. Instead, its DNA becomes integrated into the bacterial chromosome, leading to lysogeny. In this state, bacteria continue to live and replicate normally.
  7. How are Bacteriophages used?
    • Bacteriophages can be used to treat bacterial infections without harming human or animal cells. They have applications in the food industry, acting as preservatives and protecting food at pre- and postharvest stages.
  8. Can lysogenic bacteria undergo induction and enter the lytic cycle?
    • Yes, lysogenic bacteria can undergo induction, where viral DNA detaches from the host’s chromosome, initiating the lytic cycle and leading to the lysis of the bacterial cell.
  9. Are bacteriophages used in combination with antibiotics?
    • Yes, bacteriophages can be used alone or in combination with antibiotics to treat bacterial infections effectively.
  10. What is a prophage in lysogeny?
  • In lysogeny, the viral DNA integrated into the bacterial chromosome is called a prophage.
  1. How do lysogenic bacteria resist infection by phages?
  • Lysogenic bacteria are resistant to infection by the same or related phages due to the presence of integrated viral DNA passed on to each daughter cell in subsequent generations.
  1. What is the size ratio between a bacteriophage and a bacterium?
  • The volume of a bacteriophage is about 1/1000 of the bacteria (host) it infects.
  1. Which bacteriophages are most studied?
  • T2, T4, and T6 are among the most studied bacteriophages.
  1. How is a bacteriophage’s attachment to the host cell initiated?
  • Attachment occurs through weak chemical linkage between the virion and the receptor site on the bacterial cell wall.
  1. What is the primary goal of using bacteriophages in the food industry?
  • Bacteriophages are used in the food industry for preservation, extending the expiration date of food, and maintaining the cleanliness of surfaces used in plant production.
  1. Can bacteriophages be harmful to human or animal cells?
  • No, bacteriophages are specific to bacteria and do not harm human or animal cells.
  1. How is induction related to lysogeny?
  • Induction is the process where the viral DNA detached from the host’s chromosome, leading lysogenic bacteria into the lytic cycle.
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Multiple Choice Questions (MCQs)

  1. What is the primary function of a bacteriophage?
    • A) To synthesize bacterial DNA
    • B) To destroy host cells
    • C) To assist bacterial replication
    • D) To generate bacterial proteins
    • Answer: B) To destroy host cells
  2. Which of the following structural symmetries do bacteriophages exhibit?
    • A) Rectangular
    • B) Cubical and helical
    • C) Spherical
    • D) Linear
    • Answer: B) Cubical and helical
  3. What is the structural composition of bacteriophages?
    • A) Lipids and proteins
    • B) Proteins only
    • C) Nucleic acid surrounded by a protein structure
    • D) Carbohydrates and nucleic acid
    • Answer: C) Nucleic acid surrounded by a protein structure
  4. Which bacteriophages are extensively studied in phage research?
    • A) T2 and T6
    • B) T4 and T6
    • C) T2 and T4
    • D) T2, T4, and T6
    • Answer: D) T2, T4, and T6
  5. What is the role of the tail in the life process of a T Bacteriophage?
    • A) Synthesizing DNA
    • B) Dissolving bacterial cell wall
    • C) Attaching to host cells
    • D) Inducing lysis
    • Answer: B) Dissolving bacterial cell wall
  6. What defines a Lytic or Virulent Phage?
    • A) Integration into bacterial chromosome
    • B) Induction of lysogeny
    • C) Causing lysis of the host cell
    • D) Synthesizing bacterial proteins
    • Answer: C) Causing lysis of the host cell
  7. In lysogeny, what is the term for the viral DNA integrated into the bacterial chromosome?
    • A) Nucleoid
    • B) Capsid
    • C) Prophage
    • D) Nucleolus
    • Answer: C) Prophage
  8. When does induction occur in lysogenic bacteria?
    • A) During attachment
    • B) During penetration
    • C) During lysis
    • D) When viral DNA detaches from the host’s chromosome
    • Answer: D) When viral DNA detaches from the host’s chromosome
  9. What is the size ratio between a bacteriophage and the bacteria it infects?
    • A) Approximately 1:10
    • B) Approximately 1:100
    • C) Approximately 1:1000
    • D) Approximately 1:10000
    • Answer: C) Approximately 1:1000
  10. What is the primary application of bacteriophages in the food industry?
    • A) Enhancing flavor
    • B) Extending the expiration date of food
    • C) Increasing nutritional content
    • D) Accelerating fermentation
    • Answer: B) Extending the expiration date of food
  11. Can lysogenic bacteria resist infection by related phages?
    • A) No, they cannot resist
    • B) Yes, they are resistant
    • C) It depends on the bacterial strain
    • D) Only during induction
    • Answer: B) Yes, they are resistant
  12. What is the primary goal of the lytic cycle in a bacteriophage?
    • A) To integrate viral DNA into the bacterial chromosome
    • B) To induce lysogeny
    • C) To synthesize bacterial proteins
    • D) To cause lysis of the host cell
    • Answer: D) To cause lysis of the host cell
  13. How are bacteriophages different from animal viruses?
    • A) They have a similar mode of entry into host cells
    • B) They exclusively enter host cells as a whole
    • C) They do not cause lysis of host cells
    • D) They lack a protein coat
    • Answer: B) They exclusively enter host cells as a whole
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Summary

Bacteriophage – Lytic and Lysogenic Phage

This tutorial dives into the world of bacteriophages, viruses that parasitize bacteria. The two primary symmetries of bacteriophages, cubical and helical, give rise to diverse structures, including regular solid icosahedral heads and rod-shaped helical forms. The tutorial explores the T phages, with a focus on T2 and T4, widely studied for their impact on Escherichia coli.

The structure of T phages, resembling a tadpole, involves hexagonal, prism-shaped heads and intricate tail structures. The life process of a T bacteriophage unfolds within bacterial cells through stages like attachment, penetration, replication, and lysis. The lytic or virulent phages induce host cell lysis, releasing new bacteriophages to initiate the cycle anew.

Contrastingly, lysogenic or temperate phages integrate viral DNA into the bacterial chromosome, a state known as lysogeny. Lysogenic bacteria, carrying prophages, exhibit resistance to related phages. The process of induction, where viral DNA detaches, marks the transition to the lytic cycle.

The tutorial also explores the practical applications of bacteriophages, highlighting their potential in treating bacterial infections. In the food industry, bacteriophages find utility in preserving food, extending shelf life, and maintaining equipment cleanliness during production.

In essence, this tutorial provides a comprehensive understanding of bacteriophages, their structures, life cycles, and practical implications in medicine and industry.

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